With the advent of the new iPad, which has a retina display, I started thinking about what exactly a retina display is. I know it has higher resolution, but what exactly is the definition of a retina display?

Retina display is a marketing term invented by Apple; it means whatever they say it means. They have said in the keynote address for the iPhone 4 that it is a display in which, when viewed by an average person at an average viewing distance, individual pixels can not be discerned.

While talking about the new display, Steve Jobs went on to detail that after a certain point the human eye fails to distinguish individual pixels. According to the Apple CEO this "magic number," when visible pixelation is no more, is around 300ppi. Therefore, with the iPhone 4's screen coming in at more than 300 pixels per inch (326ppi) the display will supposedly always looks smooth and crisp, with no jaggies in sight.

Wikipedia was citing another source with that quote.
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jabergMar 20 '12 at 19:16

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How many levels of nested citations would you like me to provide?
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AJ.Mar 20 '12 at 19:16

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When the only thing you pull from a source is a quote from another source, it is proper to cite the original source—particularly when Wikipedia did the right thing and provided you with a direct link to that source.
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jabergMar 20 '12 at 19:28

A retina display is one that equals or surpasses the human eye in resolution - in other words where the resolution of the display is higher than the visual acuity of the eye. Specifically the fovia centralis which is the central region of the retina with the highest visual acuity, or sharpness.

The fovia centralis in the typical human eye can discern two lines as separate when they are more that 57 arcseconds apart, taking the eye as the center of the angle. That's 0.16 degrees apart. For instance, hold your hand at arms length and examine the hairs. You'll find that hairs that are very close together will appear to be one hair, but as you bring your hand closer you'll notice there are two or more hair where previously you thought there was only one.

Apple has determined (or decided...) that people hold their phones about 10" (25.4cm) from their eyes. At that distance the portion of the eye with highest visual acuity (fovia centralis) cannot discern two pixels that are 3 thousandths of an inch apart as separate pixels. The ipad is held about 15" away from your eyes, and at that distance your eye cannot discern two pixels that are 4 thousandths of an inch apart as separate pixels.

Therefore, at the ideal viewing distance, the resolution of these "retina" devices equals or exceeds the visual acuity of the portion of the human eye with the highest visual acuity - meaning that at those distances one cannot tell two adjacent pixels apart, and it appears to be one seamless image composed of continuous tones as one sees in nature, rather than a digital device composed of discrete lights.

This does mean that at a certain distance your monitor pixels will exceed your eye's visual acuity. The distinction is that there's a typical or ideal viewing distance for each display device, be it a monitor, TV, theater, or billboard. If, at that distance, the device exceeds the visual acuity of the fovia centralis, then it could be termed a retina display in the way apple is using the term. A simple trademark search for retina and retina display suggests that Apple has not trademarked this term, and none of their advertising copy has trademark near the term, so others may be able to use this term to describe their products in other ways.

@gentmatt I'm referring to Apple's earlier assertion: "While talking about the new display, Steve Jobs went on to detail that after a certain point the human eye fails to distinguish individual pixels. According to the Apple CEO this "magic number," when visible pixelation is no more, is around 300ppi." which was originally at 12 inches. Do the math (300dpi over 12 inches involving an arctan) and you get about 57 arcseconds, or 0.16 degrees. Recent comments suggest they are going bigger, and I suspect they are getting ready for a bigger iPhone 5 display, which may require 0.19 degrees...
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Adam DavisMar 20 '12 at 20:35

That's just great if apple gives an unspecific definition and later changes it. So they used to say that 12" is the normal distance to use the iPhone (2007) but now it's 10" (keynote march 2012). Maybe they did this to cheat a bit about calling the iPad's display a retina display? I'll have to look into that.
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gentmattMar 20 '12 at 20:43

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@gentmatt It's all marketing anyway. There are some metrics for how closely packed the cones are in the fovia centralis (they are optimally hexagonally packed) but everyone's eyes are slightly different in terms of size, lens, etc, so there's no universally accepted visual acuity for the human eye - just a rather large range. Personally I'm just glad that someone is pushing for this. We've already exceeded our ears ability to hear (44.1kHz at 16 bits is pretty much it, although exceptional listeners need 96kHz at 24 bits), so doing this for displays is something I've wanted for a long time.
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Adam DavisMar 20 '12 at 20:57

It has to do with normal viewing distance. The new MacBook Pro has a lower dpi than the iPad, which has a lower dpi than the iPhone, but you will normally view them from respectively farther distances. At that distance it should be impossible to discern individual pixels. I wonder how many HDTVs technically count as "retina displays" if you consider the normal viewing distance to be from across the room.

But note that if you stick even the iPhone right in your face you can glimpse individual pixels.